I looked at your scene and image files. Your ZIP file didn’t include the SHORT_GRASS_BASE image, so I substituted a simple grass texture I had. I don’t think that makes a difference in my recommendations.
First of all, I would use HDRI lighting if you can. You can get free HDR images from HDRI HAVEN. If you go into the WORLD shader editor, you can create a simple HDRI lighting setup:
The next thing I would recommend is downloading Blender Guru’s Blender Guru’s Uber Mapping node. It will make your tiled textures look much less repetitive. I would also recommend not plugging your color information directly into a Mix Shader. Always go through the correct shader node. For example, your Short Grass shader looks like this:
and renders like this:
By converting to HDR lighting and converting your shader to look like this:
The render looks much better:
You may want to think of adding bump and roughness information to the shaders as well (but that depends on how cartoonish you want it to look).
Your Main Plane shader also plugs color information directly into a shader node.
I would recommend changing it to something like this:
Notice that I’ve used the Uber Mapping node to hide repeat textures and I’ve changed your dirt mask to use Non-color data since it is a mask and not color information.
As far as your grass particle systems go, you may want to modify their orientation so they stick up properly from the ground. There are lots of videos on YouTube for how to do this properly. Making the grass smaller and adding some randomness to the particle size will also improve the look.
Your tree and leaves have problems similar to your ground. Your bark texture uses a principled shader, but your Specular and Normal channels are set to sRGB and not non-color data. You also need a Normal Map node between your normal texture and the BSDF. I would also recommend using the Roughness channel instead of Specular.
This is what your tree looks like using your shaders:
Your shader setup:
This is your leaf shader:
And this is what I recommend changing it to:
You’ll notice that I used a color ramp to create an alpha channel to isolate the leaves.
The resulting tree looks like this:
Let me know if this helps or if you need more information.